Power device cassette with auxiliary emitter contact

ABSTRACT

A press pack module includes a collector module terminal, an emitter module terminal, a gate module terminal, and an auxiliary module terminal. Each IGBT cassette within the module includes a set of shims, two contact pins, and an IGBT die. The first contact pin provides part of a first electrical connection between the gate module terminal and the IGBT gate pad. The second contact pin provides part of a second electrical connection between the auxiliary module terminal and a shim that in turn contacts the IGBT emitter pad. The electrical connection between the auxiliary emitter terminal and each emitter pad of the many IGBTs is a balanced impedance network. The balanced network is not part of the high current path through the module. By supplying a gate drive signal between the gate and auxiliary emitter terminals, simultaneous IGBT turn off in high speed and high current switching conditions is facilitated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority under 35U.S.C. §120 from, nonprovisional U.S. patent application Ser. No.14/054,704 entitled “Power Device Cassette With Auxiliary EmitterContact,” filed on Oct. 15, 2013, now U.S. Pat. No. 9,177,943, thesubject matter of which is incorporated herein by reference.

TECHNICAL FIELD

The described embodiments relate generally to high power semiconductordevice modules having cassettes.

BACKGROUND INFORMATION

FIG. 1 (Prior Art) is a sectional perspective diagram of a high powerInsulated Gate Bipolar Transistor (IGBT) press pack module 1. The module1 includes a disc-shaped bottom plate member 2 of nickel-plated copper.The disc-shaped bottom plate member 2 has a set ofhorizontally-extending channels cut into its upper surface, and also hasa set of vertically-extending channels cut into its upper surface. As aresult of the perpendicularly intersecting channels cut down into theupper surface, the bottom plate member 2 has an array of upward facingpedestals. When viewed from the top perspective, each pedestal has asubstantially square planar upper surface. The bottom surfaces of thechannels form a two-dimensional grid-shaped surface at the base level 3of the pedestals (see FIG. 2). A Printed Circuit Board (PCB) 4 isprovided that has a circular outer periphery and is of approximately thesame diameter as the disc-shaped bottom plate member 2. The disc-shapedPCB 4 also has a set of substantially square cutouts, where the shape ofeach cutout corresponds to a corresponding one of the pedestals, so thatthe disc-shaped PCB 4 can be fit down over the pedestals. When the PCB 4is fit down over the pedestals and is in place, the bottom surface 5 ofthe PCB 4 rests on the grid-shaped surface 6 of the bottom plate member2 at the bottom of the channels. Each pedestal of the bottom platemember 2 sticks up through a corresponding cutout in the PCB 4.

FIG. 2 (Prior Art) is a cross-sectional side-view diagram of one of thepedestals 9 of the bottom plate member 2. The PCB 4 that extends aroundthe pedestal 9 is seen in cross-section. The bottom surface 5 of the PCBis seen to be resting on the grid-shaped surface 6 of the bottom platemember 2 at the base level 3 of the pedestal 9.

FIG. 3 (Prior Art) is an exploded perspective view of a holding assembly7 sometimes referred to as a “cassette”. Cassette 7 includes afour-sided die locator frame 8 made of injection molded insulativeplastic. The four-sided frame 8 is open on the bottom and is open on thetop so that the frame can fit down over pedestal 9. A silver shim 10 ofthe cassette assembly fits down into the top of the frame 8 as shown inFIG. 3 and is held in place in the lateral dimension by the frame 8 sothat the bottom surface of the silver shim 10 is in contact with theupper square planar surface 11 of the pedestal 9. A first molybdenumshim 12 having a corner cutout fits down onto the upper surface of thesilver shim 10. An IGBT die 13 is placed with its frontside 14 down ontothe upper surface of the first molybdenum shim 12 so that an emittercontact pad on the frontside 14 of the die 13 is in contact with the topsurface of the first molybdenum shim 12. The first molybdenum shim 12has a corner cutout area so that a spring-loaded contact pin 15 canextend up through the plane of the first molybdenum shim 12 and makecontact with a gate contact pad on the downward facing frontside 14 ofthe IGBT die 13. A second molybdenum shim 16 is placed down onto thebackside 17 of the IGBT die 13. The backside of the IGBT die 13 is ametalized collector pad of the IGBT. The second molybdenum shim 16therefore makes contact with the collector pad of the IGBT die. Thestack of shims 10, 12, 16 and the IGBT die 13 is also seen inperspective view in FIG. 2.

One such cassette is fitted over each of the upward extending pedestalsof the bottom plate member 2. A disc-shaped top plate member 18 ofnickel-plated copper (see FIG. 1) then fits down onto the tops of thecassettes. The flat bottom surface of the top plate member 18 pressesdown onto the tops of the cassettes, thereby compressing thespring-loaded contact pins of all the cassettes, and forcing goodmechanical and electrical contact within each cassette between thebottom of the top plate member, the stack of shims, the IGBT die, andthe pedestal of the bottom plate member. Good mechanical and electricalcontact is also maintained between the gate pad on the IGBT die, thecontact pin, and a metalized track on the top of the PCB 4.

As shown in FIG. 1, the top plate member 18 fits into and is attached toa disc-shaped top lid 19. Top lid 19 is a disc of stamped metal. The toplid 19 and the top plate member 18 may, for example, be brazed together.Likewise, the bottom plate member 2 fits into and is attached to adisc-shaped bottom lid 20. Bottom lid 20 is a disc of stamped metal. Thebottom lid 20 and the bottom plate member 2 may, for example, be brazedtogether. A ceramic collar 21 is attached to the bottom lid 20. A metalflange 26 is attached to the top rim of the ceramic collar 21. Duringassembly of the press pack module, the top lid/top plate member assemblyis pressed down onto the remainder of the press pack module, and the toplid 19 is welded to the flange 26.

The press pack module 1 has a collector terminal 22, an emitter terminal23, and a gate terminal 24. The collector terminal 22 is an extension ofthe top lid 19. Within the assembly, the backside collector metal of theIGBTs are electrically coupled through the second molybdenum shims, andthrough the copper top plate member 18, to the top lid 19. The collectorterminal 22 serves as a sensing terminal to the top lid 19. Within theassembly, the frontside emitter pads of the IGBTs are coupled throughthe first molybdenum shims and the silver shims down to the solid copperbottom plate member 2, and to the bottom lid 20. The emitter terminal 23serves as a sensing terminal to the bottom lid 20. Within the assembly,the gate pads of the IGBTs are coupled through the spring-loaded contactpins down to a metalized track on the PCB 4, and laterally through themetalized track of the PCB 4 across the module to the inside edge of theceramic collar, via a clip 25 through a hole in the ceramic collar 21,and to the gate terminal 24. Many IGBT dice may be provided in this wayin one press pack module.

Although the cassette is described here as retaining an IGBT die, acassette can also retain other types of semiconductor dice. A cassettecan, for example, contain a diode die. U.S. Pat. No. 6,678,163 disclosesa module involving twenty-eight IGBT dice and nine diode dice. Thedescription above of a press pack module is simplified and is presentedfor background purposes. There are several different variations andtypes of press pack modules involving cassettes. Rather than using a PCBto provide connection between the gates of the IGBTs and the gateterminal, a preformed insulating insert and stamped copper sheet can beused within the module for gate signal distribution purposes. Foradditional information on press pack modules and cassettes, see: U.S.Pat. Nos. 6,678,163 and 6,303,974 (the subject matter of which areincorporated herein by reference).

SUMMARY

A high power semiconductor device press pack module includes a matrix ofcassettes. Each of at least one of the cassettes includes a high powersemiconductor device die and two contact pins. Each of these cassetteshaving contact pins is compressed between a metal top plate member ofthe module and pedestals of a metal bottom plate member of the module sothat four electrical connections are established.

A first electrical connection extends from a first pad (for example, agate pad) of each semiconductor device die, through one of the contactpins of the cassette that holds the die, and to a first module terminal.Where the first pad is a gate pad of an IGBT die, the IGBT die mayinclude an on-chip resistor between the IGBT gate and the IGBT gate pad.

A second electrical connection extends from a second pad (for example,an emitter pad) of each semiconductor device die, through the othercontact pin of the cassette that holds the die, optionally through aresistor, and to a second module terminal. The first module terminal maybe a gate terminal of the module. The second module terminal may be anauxiliary emitter terminal of the module.

A third electrical connection extends from a third pad (for example, acollector pad) of each semiconductor device die, through the top metalplate member, and to a top circular contact surface of the module. Athird terminal of the module, that serves as a sensing terminal, is alsocoupled to the upper circular contact surface of the module. Where thedie is an IGBT die, the top circular contact surface of the moduleserves as the main collector terminal of the module.

A fourth electrical connection extends from the second pad (for example,the emitter pad), through the metal bottom plate member, and to a bottomcircular contact surface of the module. A fourth terminal of the module,that serves as a sensing terminal, is also coupled to the bottomcircular contact surface of the module. Where the die is an IGBT die,the bottom circular contact surface of the module serves as the mainemitter terminal of the module.

Accordingly, in a first novel aspect, the high power semiconductordevice press pack module includes cassettes and has four moduleterminals: a collector module terminal, an emitter module terminal, agate module terminal, and an auxiliary emitter module terminal. Some ofthe cassettes hold three terminal devices such as IGBTs, whereas othersof the cassettes may hold two-terminal devices such as power diodes. Inone particular embodiment, each cassette that holds a power IGBT dieincludes a first molybdenum shim that is in contact with the emitter padon the frontside of the IGBT die. The auxiliary emitter terminal of themodule is coupled to the emitter pad of each IGBT die via a conductivefeedthrough that extends through a hole in a ceramic collar of themodule, across a multi-layer printed circuit board to a bond pad of theprinted circuit board, through a surface mount resistor to another bondpad of the printed circuit board, across the printed circuit board to acontact pad, up through a contact pin of the cassette that holds theIGBT die to a location on the bottom surface of the first molybdenumshim, and through the first molybdenum shim to the emitter pad on thefrontside of the IGBT die. The auxiliary emitter terminal is connectedto the emitter pad of each IGBT die in parallel in this fashion. Theprinted circuit board is laid out so that the auxiliary terminal toemitter pad connections are a branched and balanced impedance network.The main current path through the module does not pass through thisauxiliary emitter balanced impedance network, but rather extends fromthe top circuit contact surface of the module (the main collectorterminal), downward through a top lid of the module, downward throughthe top metal plate member, downward through a second molybdenum shim,downward through a layer of sintered silver to the collector pad on thebackside of the IGBT die, downward through the IGBT die to the emitterpad on the frontside of the IGBT die, downward through the firstmolybdenum shim, downward through a silver shim, downward through thebottom metal plate member, downward through a bottom lid of the module,and to the bottom circular contact surface (the main emitter terminal)of the module. The press pack module receives its control signal betweenthe gate module terminal and the auxiliary emitter module terminal (asopposed to between the gate module terminal and the main emitterterminal), so that variations in the reactances between the emitter padsof the individual IGBT dice and the emitter module terminal under highspeed and high current turn off conditions do not cause some of the IGBTdice to turn off after others.

In a second novel aspect, the power semiconductor device die in acassette of a press pack module has a second metal shim bonded to thedie backside by a layer of sintered metal. The layer of sintered metalmay be a layer of sintered silver microparticles or silvernanoparticles. The die, the layer of sintered metal, and the secondmetal shim together form a sintered assembly that is assembled first,and is then incorporated into the cassette. The cassette is compressedbetween a metal top plate member of the module and a metal bottom platemember of the module such that the backside of the sintered assembly ispressed against the metal top member, and such that the frontside of thesintered assembly is pressed against the first metal shim. Within thecassette, a central portion of the frontside surface of the die iscontacted by the top of a first metal shim, but there is no shim incontact with the peripheral portion of the frontside surface of thepower semiconductor device die. Despite there being no shim in contactwith the peripheral portion of the frontside of the die, the peripheralportion of the die is in good thermal contact with the metal top platemember through the layer of sintered metal and through the second metalshim that is bonded to the backside of the die.

The first and second novel aspects described above can be usedindependently of one another. A press pack module can employ the novelauxiliary emitter terminal without using the novel sintered backsideshim attach. Alternatively, the novel sintered backside shim attach canbe employed in a press pack module than has no auxiliary emitterterminal.

Further details and embodiments and methods are described in thedetailed description below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 (Prior Art) is a sectional perspective diagram of a high powerIGBT press pack module.

FIG. 2 (Prior Art) is a cross-sectional side view of a cassette in thepress pack module of FIG. 1.

FIG. 3 (Prior Art) is an exploded view of the cassette of FIG. 2.

FIG. 4 is a sectional perspective diagram of a high power IGBT presspack module in accordance with one novel aspect.

FIG. 5 is a cross-sectional side view showing a cross-section of theceramic collar at the left of FIG. 4.

FIG. 6 is a cross-sectional side view showing a cross-section of theceramic collar at the right of FIG. 4.

FIG. 7 is a perspective exploded view of the press pack module of FIG.4.

FIG. 8 is a perspective exploded view of a cassette that includes twospring-loaded contact pins in accordance with a first novel aspect.

FIG. 9 is a cross-sectional diagram of one of the spring-loaded contactpins of FIG. 8.

FIG. 10 is a top-down diagram showing the cassette of FIG. 8 disposed ona pedestal.

FIG. 11 is a cross-sectional side view taken along line A-A of FIG. 10.

FIG. 12 is a perspective exploded view of a cassette that includes adiode die and no spring-loaded contact pins.

FIG. 13 is a top-down diagram of the top metal layer of the PCB of themodule of FIG. 4.

FIG. 14 is a top-down diagram of the second metal layer of the PCB ofthe module of FIG. 4.

FIG. 15 is a top-down diagram of the third metal layer of the PCB of themodule of FIG. 4.

FIG. 16 is a top-down diagram of the bottom metal layer of the PCB ofthe module of FIG. 4.

FIG. 17 is a diagram that shows the locations of the resistors in theauxiliary emitter signal distribution network of the module of FIG. 4.

FIG. 18 is a simplified schematic diagram of the IGBTs and diodes of theconventional press pack module of FIG. 1.

FIG. 19 is a simplified schematic diagram of the IGBTs and the diodes ofthe novel press pack module of FIG. 4.

FIG. 20 is a flowchart of a method in accordance with the first novelaspect.

FIG. 21 is a cross-sectional diagram of a cassette on a pedestal in theconventional press pack module of FIG. 1.

FIG. 22 is a cross-sectional diagram that illustrates how a conductiveshim is bonded to the backside of the semiconductor device die with alayer of sintered silver in accordance with the second novel aspect.

FIG. 23 is a flowchart of a method in accordance with the second novelaspect.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. In this patent document, if a first object is referred to asbeing disposed “over” or “on” a second object, it is to be understoodthat the first object can be directly on the second object, or anintervening object may be present between the first and second objects.Similarly, relational terms such as “above”, “below”, “top”, “bottom”,“upper” and “lower” are used herein to describe relative orientationsbetween different parts of the structure being described, and it is tobe understood that the overall structure being described can actually beoriented in any way in three-dimensional space.

FIG. 4 is a sectional perspective diagram of a high power Insulated GateBipolar Transistor (IGBT) press pack module 50. In a first novel aspect,the module 50 has an auxiliary emitter terminal, where the auxiliaryemitter terminal is coupled in parallel by a branched balanced impedancenetwork to a lower shim in contact with the emitter pad of each IGBT ofthe module 50. A cassette in the press pack module has two contact pins.In one example, one contact pin makes direct physical contact to a firstpad on the die of the cassette, whereas the other contact pin makesphysical contact with a shim that in turn is in contact with a secondpad of the die. In a second novel aspect, within a cassette of themodule 50 an upper shim is bonded to the backside of the semiconductordevice die with a layer of sintered metal.

In the example of FIG. 4, the branched balanced impedance networkprovides a highly uniform gate drive signal propagation delay from thepackage terminal to each individual semiconductor die. The branchedbalanced impedance network also avoids negative gate voltage feedbackdue to induced voltages across the emitter power connection selfinductances. In addition, the branched balanced impedance networkprovides high immunity to electrostatic and electromagnetic fields byappropriately designing the PCB board metal layout, particularly, thedimensions of the individual metal contact runners.

Module 50 includes a bottom lid 51 of metal, a disc-shaped bottom platemember 52 of nickel-plated copper, a ceramic collar 53, a collar flangeof metal 54, a disc-shaped Printed Circuit Board (PCB) 55, a matrix ofcassettes including cassette 56, a disc-shaped top plate member 57, atop lid 58 to metal, a gate terminal 59, a clip 64 and feedthroughconnection (not shown) for the gate terminal, an auxiliary emitterterminal 61, and a clip (not shown) and feedthrough (not shown) for theauxiliary emitter terminal 61. The clip and feedthrough connection forthe auxiliary emitter terminal 61 are not seen in the particular view ofFIG. 4.

The bottom lid 51 is a stamped disc-shape piece of sheet metal. Thebottom plate member 52 is a thick, disc-shaped piece of machinednickel-plated copper. The bottom plate member 52 and the bottom lid 51are shaped to engage one another in a friction fit fashion so that thebottom plate member 52 is fixed to the bottom lid 51. The bottom platemember 52 and the bottom lid 51 may be permanently joined together bybrazing. The bottom lid 51 has an extension that forms an emitterterminal 62 of the module. The emitter terminal 62 is typically used forsensing. The main emitter terminal that handles the bulk of current flowis the bottom (downward facing in the illustration) circular contactsurface of the bottom lid 51.

Similarly, the top lid 58 is a stamped disc-shaped piece of sheet metal.The top plate member 57 is a thick, disc-shaped piece of machinednickel-plated copper. The top plate member 57 and the top lid 58 areshaped to engage one another in a friction fit fashion so that the topplate member 57 is fixed to the top lid 58. The top plate member 57 andthe top lid 58 may be permanently joined together by brazing. The toplid 58 has an extension that forms a collector terminal 63 of themodule. The collector terminal 63 is typically used for sensing. Themain collector terminal that handles the bulk of current flow is the top(upward facing in the illustration) circular contact surface of the toplid 58.

The disc-shaped bottom plate member 52 has a set ofhorizontally-extending channels cut into its upper surface, and also hasa set of vertically-extending channels cut into its upper surface. As aresult of the perpendicularly intersecting channels cut down into theupper surface, the bottom plate member 52 has an array of upward facingpedestals. When viewed from the top perspective, each pedestal has asquare upper surface or a substantially square upper surface. The uppersurface of the pedestal is planar. The bottom surfaces of the channelsform a two-dimensional grid-shaped surface at the base level of thepedestals. The PCB 55 has a circular outer periphery and is ofapproximately the same diameter as the disc-shaped bottom plate member52. The disc-shaped PCB 55 has a set of cutouts, where the shape of eachcutout corresponds to a corresponding one of the pedestals. Due to thecutouts, the disc-shaped PCB 55 can be fit down over the pedestals. Whenthe PCB 55 is fit down over the pedestals and is in place as seen inFIG. 4, the bottom surface of the PCB 55 rests on the grid-shapedsurface of the bottom plate member 52 at the bottom of the channels.Each pedestal of the bottom plate member 52 sticks up through acorresponding cutout in the PCB 55. The PCB 55 is screwed to the bottomplate member 52 to fix it in place. A cassette is disposed on and fitsover each of the pedestals. As explained in further detail below, thereare two types of cassettes in the particular module 50 of FIG. 4. Thefirst type of cassette contains an IGBT die. The second type of cassettecontacts a diode die.

There is a first laterally-extending hole in the ceramic collar 53. Aconductive feedthrough (not shown) is provided in this hole. Theconductive clip 64 provides an electrical connection between a gate bondpad on the top surface of the PCB 55 and the conductive feedthrough. Thegate terminal 59 is connected to the conductive feedthrough on theoutside of the ceramic collar 53. There is therefore an electricalconnection between the bond pad on the surface of the PCB 55 and thegate terminal 59 on the outside of the module 50.

Likewise, there is a second laterally-extending hole in the ceramiccollar 53. A conductive feedthrough (not shown) is provided in thishole. The conductive clip (not shown) for the auxiliary emitterconnection provides an electrical connection between an aux emitter bondpad on the top surface of the PCB 55 and the conductive feedthrough. Theauxiliary emitter terminal 61 is connected to the conductive feedthroughon the outside of the ceramic collar 53. In the particular perspectiveview of FIG. 4, only the auxiliary emitter terminal 61 of thisconnection is in view.

The flange 54 is attached to the upper rim of the ceramic collar 53 andthe bottom of the ceramic collar 53 is attached to the bottom lid 51 tohave the relation shown in FIG. 4. The PCB 55 is fitted down over theupward projecting pedestals of the bottom plate member 52. A gate signalinterconnect network of the PCB 55 is connected, via the clip 64 to theconductive feedthrough (that passes through the ceramic collar) for thegate terminal 59. An auxiliary emitter signal interconnect network ofthe PCB is connected, via another clip, to the conductive feedthrough(that passes through the ceramic collar) for the auxiliary emitterterminal 61. The cassettes are placed over their respective pedestals.The top lid 58 and top plate member 57 sub-assembly is placed down ontothe tops of the cassettes, and is pressed down to compress spring-loadedcontact pins within the cassettes. The peripheral edge of the top lid 58is then welded to the flange 54, thereby securing the cassettes on theirpedestals.

FIG. 5 is an expanded view of a part of FIG. 4. The cross-section of theceramic collar 53 at the left of FIG. 4 is seen in more detail in FIG.5.

FIG. 6 is an expanded view of another part of FIG. 4. The cross-sectionof the ceramic collar 53 at the right of FIG. 4 is seen in more detailin FIG. 6.

FIG. 7 is an exploded perspective view of the module 50 of FIG. 4. InFIG. 7, the bottom plate member 52 that includes the pedestals is shownin a sub-assembly along with the ceramic collar 53, flange 54, andterminals 59 and 61.

FIG. 8 is an exploded view of the cassette 56 of FIG. 4. This cassetteis of the first type and contains an IGBT die 65. Cassette 56 includes afour-sided die locator frame 66 made of injection molded insulativeplastic. The four-sided frame 66 is open on the bottom and is open onthe top so that the frame can fit down over a pedestal. A silver shim 67of the cassette assembly fits down into the top of the frame 66 as shownin FIG. 8. If the frame is not in place of a pedestal, then the edges ofthe silver shim 67 may rest on ledges on the inside of the frame 66 sothat the silver shim does not fall through the opening in the frame. Thesilver shim 67 is held in place in the lateral dimension by the sides ofthe frame. When the cassette 56 is in place on a pedestal, the bottomsurface of the silver shim 67 is in contact with the upper square planarsurface of the pedestal. A first molybdenum shim 68 having a cornercutout fits down onto the upper surface of the silver shim 67. The IGBTdie 65 is placed with its frontside 69 down onto the upper surface ofthe first molybdenum shim 68 so that an emitter contact pad on thefrontside 69 of the die 65 is in contact with the top surface of thefirst molybdenum shim 68. The first molybdenum shim 68 has a cornercutout area so that a first spring-loaded contact pin 70 can extend upthrough the plane of the first molybdenum shim 68 and make directcontact with a gate contact pad on the downward facing frontside 69 ofthe IGBT die 65. The first molybdenum shim 68 is smaller than the IGBTdie 65, so that a peripheral portion of the downward facing frontside 69surface of the IGBT die 65 is not touching any shim. A secondspring-loaded contact pin 71 makes contact with the first molybdenumshim 68 and through the first molybdenum shim 68 is in electricalcontact with the emitter contact pad on the frontside 69 of the IGBT die65. A second molybdenum shim 73 is bonded to the backside 74 of the IGBTdie 65 by a layer of sintered metal 75. In a preferred embodiment, thelayer of sintered metal 75 is a layer of sintered silver microparticlesor silver nanoparticles. The backside 74 of the IGBT die 65 is ametalized collector pad of the IGBT. The second molybdenum shim 73 istherefore in electrical contact with the collector pad of the IGBT die65. The first contact pin 70 slides into and is retained by a firstcylindrical channel 76 in the frame 66. The second contact pin 71 slidesinto and is retained by a second cylindrical channel 77 in the frame 66.The frame 66 has upward extending column projections 78 that keep thestack of shims and the IGBT die 65 properly aligned above the top of thepedestal.

FIG. 9 is a cross-sectional diagram of the spring-loaded contact pin 71.The contact pin 71 has a first end 79 and a second end 80. The first end79 is to make physical contact with the first molybdenum shim 68, andthereby to make electrical contact with the emitter pad of the IGBT. Thesecond end 80 is to make physical contact with a metal contact pad on anupper surface of the PCB 55. The head 81 of the contact pin slidinglyengages a head collar 82. If there is no pressure on the head 81, thenthe head 81 is pushed out to its outer extent by a spring 83. If thereis pressure on the head 81, then the head 81 can be pushed into the headcollar 82 by compressing the spring 83. The spring 83 is retained by asleeve 84. The gate contact pin 70 is of the same construction as isemitter contact pin 71, except that the gate contact pin 70 may be madelonger by the thickness of the first molybdenum shim 68.

FIG. 10 is a simplified top-down diagram that illustrates how thecassette 56 of FIG. 8 is fitted onto a pedestal 85. Dashed line 86identifies the outer surface of the frame 66. Dashed line 87 identifiesthe inner surface of the frame 66. Reference numerals 66A and 66Bidentify the ledges upon which the silver shim 67 can rest if thecassette 56 is not in place on a pedestal. The cassette 56 is positionedover the edge 88 of a cutout in the PCB 55 such that the longitudinalaxis of the spring-loaded contact pin 71 makes physical contact with acontact pad 89 on the upper surface of the PCB 55. The contact pad 89 isconnected via the PCB 55 and series resistors, and a clip, and aconductive feedthrough, to the auxiliary emitter terminal 61. Thecassette 56 is positioned such that the longitudinal axis of thespring-loaded contact pin 70 makes physical contact with a contact pad90 on the upper surface of the PCB 55. The contact pad 90 is connectedvia the PCB 55, and the clip 64, and a conductive feedthrough, to thegate terminal 59.

FIG. 11 is a cross-sectional diagram taken along sectional line A-A ofFIG. 10. The stack of shims and the IGBT die 65 is compressed betweenthe bottom surface 96 of the top plate member 57 and the planar uppersurface 94 of the pedestal 85. The contact pins (not shown in thiscross-section A-A) press down on the top surface 95 of the PCB 55. Thebottom surface 91 of the PCB 55 is therefore pressed downward onto thegrid-shaped surface 92 at the base level 93 of the pedestal 85.

FIG. 12 is an exploded view of a cassette 97 of the second type.Cassette 97 contains a diode die 98. The cassette 97 of FIG. 12 is ofsimilar construction to the cassette 56 of FIG. 8 except that thecassette 97 does not involve contact pins. The frame 99 has nocylindrical channels for retaining contact pins, and the cassetteassembly includes no contact pins. A silver shim 100 of the cassetteassembly fits down into the top of the frame 99 as shown in FIG. 12 andis held in place in the lateral dimension by the frame 99 so that thebottom surface of the silver shim 100 is in contact with the uppersquare planar surface of a pedestal. A first molybdenum shim 101 fitsdown onto the upper surface of the silver shim 100. The diode die 98 isplaced with its frontside 102 down onto the upper surface of the firstmolybdenum shim 101 so that an anode contact pad on the frontside of thedie is in contact with the top surface of the first molybdenum shim 101.A second molybdenum shim 103 is bonded to the backside 104 of the diodedie 98 by a layer of sintered silver 105. The backside of the diode die98 is a metalized cathode pad of the diode. The second molybdenum shim103 is therefore in electrical contact with the cathode pad of the diodedie 98. The frame 99 has upward extending column projections 106 thatkeep the stack of shims and the diode die 98 properly aligned above thetop of the pedestal.

FIG. 13 is a simplified top-down diagram of the top metal layer of thePCB 55. FIG. 14 is a simplified top-down diagram of the metal layer 2 ofthe PCB 55. FIG. 15 is a simplified top-down diagram of the metal layer3 of the PCB 55. FIG. 16 is a simplified diagram of the bottom metallayer of PCB 55. FIG. 17 shows where surface mount resistors are mountedto the top layer of the PCB. The surface mount resistors of FIG. 17 arethe resistors in the auxiliary emitter balanced network.

In FIG. 13, the dashed line 86 is the same dashed line 86 in FIG. 10that shows the outer periphery of the frame 66 of the cassette 56 thatholds IGBT die 65. Reference numeral 85 identifies the pedestal 85. Thecassette 56 is not shown, but the line labeled 86 is provided to showhow the cassette 56 is positioned on the pedestal 85. Reference numeral88 identifies the edge of the cutout in the PCB 55.

A first connection extends from the gate pad 116 on the top of the PCB55, to pad 117, downward through a conductive via to metal layer 2 ofFIG. 14, and from location 118, across conductor 119, to location 120,and back up through a conductive via to a pad 121 on the top of the PCB55, and then across the top layer of metal to a pad 113, through asurface mount resistor 114 to pad 115, and then across the top layer ofmetal to the contact pad 90. As explained above, the spring-loadedcontact pin 70 of the cassette 56 provides electrical contact from thecontact pad 90 up to the gate pad on the IGBT die 65. Within the IGBTdie 65, there is an on-chip resistor in the current path to the actualgate of the IGBT. There are many such first electrical connections thatextend in parallel from the gate pad 116 on the top of the PCB 55 to thegates of the various IGBT dice in the various cassettes of the module50. Gate pad 116 on the top of the PCB 55 is shown in the detail of FIG.5. These first connections to the gates of the IGBTs are provided as animpedance matched balanced network.

A second electrical connection extends from the auxiliary emitter pad107 on the top of the PCB 55, to pad 108 of FIG. 13, downward through aconductive via to metal layer 3 of FIG. 15, and from location 109,across conductor 110, to location 111, and back up through a conductivevia to a pad 112 on the top of the PCB 55, and then across the top layerof metal to a pad 113, through a surface mount resistor 114 to pad 115,and then across the top layer of metal to the contact pad 89. Theresistor 114 is shown on FIG. 17. As explained above, the spring-loadedcontact pin 71 of the cassette 56 provides electrical contact from thecontact pad 89 up to the first molybdenum shim 68 and the emitter pad onthe IGBT die 65. There are many such second electrical connections thatextend in parallel from the auxiliary emitter pad 107 of the PCB 55,through surface mount resistors, and to the emitters of the various IGBTdice in the various cassettes of the module 50. These second connectionsare provided as an impedance matched balanced network.

As mentioned above, some of the cassettes include IGBT dice whereasother cassettes include diode dice. In FIG. 13, a cutout, such as cutout88, that is not entirely square (but rather has two inwardly extendingcorner regions with contact pads for engaging spring-loaded contactpins) is at a location where a cassette having an IGBT die is disposed.In FIG. 13, a cutout, such as cutout 130, that is entirely square is ata location where a cassette having a diode die is disposed. Accordingly,module 50 of FIG. 4 has thirty IGBT cassettes and fourteen diodecassettes.

FIG. 18 is a simplified schematic of the IGBTs in the press pack module1 of FIG. 1. The IGBT symbols 122-127 represent the IGBT dice of themodule 1 and the diode symbols 128 and 129 represent the diode dice ofthe module 1. Resistor symbols 131-136 represent on-chip resistors inthe gate signal distribution network of the module 1. The module 1 ispressed between two bus bars 161 and 162. The IGBTs of the module 1 areturned on and off together by supplying a gate drive signal between thegate terminal 24 and the bottom circular contact surface of the bottomlid. Although some current flows into the gate terminal 24, the maincurrent path is to the right through the upper bus bar 161, downwardthrough the top circular contact surface of the top plate member/toplid, through the top plate member/top lid, through the IGBTs, throughthe bottom plate member/bottom lid, out of the bottom circular contactsurface of the bottom plate member/bottom lid, through the bus bar 162back to the left. The module 1 is a very high current device. The module1 may, for example, be conducting 4000 amperes from the collectorterminal to the emitter terminal. The IGBTs are then to be turned offrapidly in, for example, one millisecond. There is some inductance ineach current path between an IGBT emitter and the left end of the busbar 162. The voltage drop across one such inductance can be given asV=LdI/dt, where the L is the inductance. Because the current to bestopped is so large, and because the turn off time is so short, thedI/dt factor is substantial. Even slight differences in the inductancesof the emitter current paths (between an IGBT emitter and the end of thebus bar 162) of the various IGBTs give rise to voltage drops acrossthese current paths that may differ by more than 100 millivolts. Theresulting slightly different emitter voltages cause some IGBTs to turnoff faster than others. Where 4000 amperes is flowing, the last IGBT toturn off may have to turn off too much of this 4000 amperes of currentand may fail or be overstressed.

FIG. 19 is a simplified schematic of the IGBTs in the module 50 of FIG.4 in accordance with a first novel aspect. The IGBT symbols 172-177represent the IGBT dice of the module 50. Diode symbols 178 and 179represent the diode dice of the module 50. Resistor symbols 144-149represent the resistors in the auxiliary emitter signal distributionpath of the module 50. Resistor symbols 163-168 represent the on-chipresistors to the actual gates of the IGBTs. Rather than turning on andoff the IGBTs using a gate drive signal between the gate terminal andthe main emitter terminal as in the example of FIG. 18, the IGBTs of themodule 50 are turned on and off by supplying a gate drive signal 169between the gate terminal 59 and the auxiliary emitter terminal 61. Themain current path from an emitter of an IGBT, downward through thebottom plate member/bottom lid 52/51, and through the bus bar 162, andto the left end of the bus bar 162 does not pass through the auxiliaryemitter spring-loaded contact spring of the cassette, but rather themain current path extends from the emitter pad on the frontside of theIGBT die, downward through the first molybdenum shim, through the silvershim, downward through the pedestal, through the remainder of the bottomplate member 52, through the bottom lid 51, out of the bottom circularcontact surface (main emitter terminal) of the module, through the busbar 162, and to the left end of the bus bar 162. The gate drive signalis applied directly across the gate-to-emitter junctions of the IGBTsthrough resistors 144-149 so that variations in the voltage dropsbetween the IGBT emitters and the emitter terminal 61 during device turnoff has less effect on the V_(GE) signals that control the IGBTs. Ascompared to the situation described above in connection with FIG. 18,the IGBTs of FIG. 19 see substantially the same gate drive signalvoltage between their gate pads and their emitter pads at the time ofturn off. As compared to the situation described above in connectionwith FIG. 18, the IGBTs of FIG. 19 turn off more uniformly at the sametime.

FIG. 20 is a flowchart of a method 200 in accordance with the firstnovel aspect. A first conductive path is provided (step 201) between afirst pad (for example, the gate pad) on the frontside of a powersemiconductor die (for example, an IGBT die) and a first terminal (forexample, the gate module terminal) of a press pack module. The firstconductive path passes through a first contact pin. A second conductivepath is provided (step 202) between a second pad (for example, theemitter pad) on the frontside of the power semiconductor die and asecond terminal (for example, the auxiliary emitter module terminal) ofa press pack module. The second conductive path passes through a secondcontact pin, and may also pass through a resistor. A third conductivepath is provided (step 203) between a third pad (for example, thecollector pad) on the backside of the power semiconductor die and athird terminal (for example, the main collector terminal) of the presspack module. The third conductive path passes through the metal topplate member of the module. A fourth conductive path is provided (step204) between the second pad on the frontside of the die and a fourthterminal (for example, the main emitter terminal) of the press packmodule. The fourth conductive path passes through the metal bottom platemember of the module. All the steps are typically performedsimultaneously during the assembly of the module. In one example, thefirst and second contact pins are the spring-loaded contact pins of FIG.8, and the press pack module is the module of FIG. 4.

FIG. 21 is a simplified cross-sectional diagram of the shim and IGBT diestack within a cassette in the press pack module 1 of FIG. 1. There arevoltage protection edge termination structures on the frontside of theIGBT die 13 at the periphery of the die. In a reverse blocking voltagemode when the edge termination structures are protecting the IGBT diefrom breakdown, there is a very large voltage across these structures.This voltage is typically several thousand volts. A piece of metaldisposed along the surface of the edge termination structure woulddestroy the high voltage protection function of the edge termination andguard ring structures. In addition, unlike the emitter pad that has arelatively thick amount of soft metal on it, the edge terminationstructures are not covered with such thick layers of metal and arerelatively fragile. Also, if mechanical pressure were put on the edgetermination structures, then the electric field distribution within thedie could be changed and the die would be susceptible to failure. Forthese reasons, these edge termination structures should not be coveredor touched by the molybdenum shim 12. When the top plate member 18presses down on the stack to compress the stack between the bottomsurface of the top plate member 18 and the top of the pedestal 9, thereis no part of the first molybdenum shim 12 underneath these peripheralparts of the IGBT die 13. Consequently there is less compressive forceon the IGBT die 13 at the die periphery. Due to the lower compressiveforce, the thermal contact between the IGBT die 13 and the top platemember 18 is not as good at the periphery of the die as it is in thecenter of the die.

FIG. 22 is a more detailed diagram of the how the backside of IGBT die65 is bonded to the second molybdenum shim 73 by the sintered silverlayer 75 in accordance with a second novel aspect. The sintered silverlayer 75 provides thermal contact between the peripheral portion 159 ofthe die 65 and the molybdenum shim 73 even if the two objects are notbeing pressed together in the cassette. Due to the better thermalcontact between the peripheral portion 159 of the die 65 and themolybdenum shim 73, there is better thermal contact to the top platemember 57 as well.

The backside of the die 65 includes an aluminum contact layer 150 to thesilicon. The aluminum layer 150 is covered by a diffusion barrier layer151 (for example, titanium). The diffusion barrier layer 151 is coveredby an optional nickel layer 152, which in turn is covered by a gold orsilver layer 153. The semiconductor portion 156, and the aluminum layer150, and the diffusion barrier layer 151, and the nickel layer 152, andthe gold or silver layer 153 are referred to together here as the IGBTdie 65.

The bottom of the molybdenum shim 73 is actually an undercoat layer 154(for example, an undercoat layer of ruthenium, nickel, or rhodium, or acombination of them) that is covered by a gold or silver layer 155. Themolybdenum portion 157, and the ruthenium layer 154, and the gold orsilver layer 155 are referred to together as the second molybdenum shim73.

Before the die 65 is placed into the frame, the die 65 is bonded to thesecond molybdenum shim 73 to form a sintered assembly 158. In oneexample, the surfaces to be joined are cleaned to remove oxides andother contaminants. Wet etching can be employed. In-situ plasma cleaningcan be employed in the case where the surface to be cleaned wasdeposited by PVD sputtering. After cleaning, a layer of commerciallyavailable nanoparticle silver paste approximately 40-80 microns thick isapplied to one of the cleaned surfaces. The molybdenum shim 73 and theIGBT die 65 are then pressed together under pressure (10-30 MPa) so thatthe nanoparticle paste is pressed between the cleaned surfaces. Thepressed together structure is then heated. As the temperature increasesto about 150° C., a thinner component of the paste evaporates. Thisresults in a somewhat more dense packing of the nanoparticles. In thepaste, the nanoparticles of silver are coated with a dispersant/binder.At about 200° C., the dispersant/binder coating separates from thesilver particles and burns out. After burn out of the organic compounds,the temperature is increased to the higher sintering temperature of 250°C. The sintering 250° C. temperature causes the silver nanoparticles todensify and to sinter together. The silver nanoparticle paste may be amAgic Paste Microbond paste, series ASP016, ASP043, ASP131 or APA859,that is commercially available from Heraeus Materials Technology GmbH &Co.KG of Hanau, Germany.

In another example, rather than applying silver microparticles ornanoparticles as a paste to the second molybdenum shim 157, the silvermicroparticles or nanoparticles are commercially available already on afoil. The silver particles on the foil are in a controlled uniformthickness. The silvered foil is placed silver side down on themolybdenum shim 157 with the silver down on the molybdenum, and then thestructure is heated and put under some pressure. As a result, the foilcan later be easily removed from the silver. With the molybdenum shimsilvered in this way, the backside of the die is placed down on thesilvered surface of the molybdenum. The die 65 and the molybdenum shim157 are pressed together under a higher temperature, and the sinteringof the layer of silver is completed such that the molybdenum and theIGBT die are bonded together.

To assemble the cassette, shims 67 and 68 are placed into the frame 66.The sintered assembly 158 is then placed into the frame 66 as shown inFIG. 22 to make the completed cassette 56. The cassette is then tested.During handling and testing, the silver shim 67 prevents the firstmolybdenum shim 68 from falling through the opening in the frame 66.Once the cassette has been tested, the cassette is placed over thepedestal 85 of the bottom plate member. The top plate member 57 is thenapplied as described above in connection with FIGS. 4-7 to make thecompleted press pack module 50. The shims 67 and 68 and the sinteredassembly 158 are compressed together as a stack between the bottomsurface of the top plate member 57 and the upper surface of the pedestal85 of the bottom plate member 52. The first molybdenum shim 68 contactsthe central portion 160 of the die 65 (the emitted pad), but theperipheral portion 159 of the die 65 (where the edge terminationstructures are) has no part of the first molybdenum shim 68 contactingit from underneath. In a typical example, the peripheral portion 159 isapproximately two millimeters wide. Even though the downward facingsurface of peripheral portion 159 of the IGBT die 65 has no part of thefirst molybdenum shim 68 contacting it from underneath, the peripheralportion 159 of the die 65 nevertheless has improved thermal contact withthe top plate member 57 as compared to the situation depicted in FIG.20.

In one example: molybdenum shim 73 is 14.5 mm wide by 1.5 mm thick; IGBTdie 65 is 14.3 mm wide by 0.7 mm thick; molybdenum shim 68 is 10.4 mmwide by 1.1 mm wide; silver shim 67 is 12.6 mm wide (max width) by 0.15mm thick; and pedestal 85 is about 10.55 mm wide by 8.7 mm high.Although the backside silver sintering of a conductive shim onto a dieis described here with IGBT die 65 as the example, the same silversintering attachment process may be applied to the other semiconductordice of the module 50. For example, the diode dice in the module 50 mayalso be bonded to their respective shims by silver sintering before theyare placed in their frames and incorporated into the module 50.

FIG. 23 is a flowchart of a method 300 in accordance with the secondnovel aspect. A sintered assembly is provided (step 301) in a cassettewithin a power semiconductor device press pack module. The sinteredassembly comprises a semiconductor device die whose backside is bondedby a layer of sintered metal to a metal shim. In one example, the die isthe die 65 of FIG. 22, the shim is shim 73 of FIG. 22, and the layer ofsintered metal is layer 75 of FIG. 22.

Although the present invention has been described in connection withcertain specific embodiments for instructional purposes, the presentinvention is not limited thereto. The auxiliary terminal describedabove, and the gate terminal described above, are each just one exampleof a control module terminal that is connected by a branched balancedimpedance network to a corresponding control pad on each of the powerdevices of a press pack module. Such a terminal and its associatedbranched balanced impedance network that connects the terminal to padson semiconductor dice in a press pack module sees general applicability.It can be used for both sensing purposes and well as device drivingpurposes. Accordingly, various modifications, adaptations, andcombinations of various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

What is claimed is:
 1. A power semiconductor device module comprising: atop plate member; a bottom plate member having a plurality of pedestals;a plurality of semiconductor device dice, wherein each of thesemiconductor device dice is positioned above a corresponding one of theplurality of pedestals between the bottom plate member and the top platemember, and wherein each of the semiconductor device dice has an emitterpad and a gate pad; an auxiliary emitter terminal, wherein the auxiliaryemitter terminal is coupled via a branched network to the emitter padsof the semiconductor device dice; and a main emitter terminal, whereinthe main emitter terminal is coupled through the pedestals to theemitter pads of the semiconductor device dice, and wherein the branchednetwork does not extend through any of the pedestals.
 2. The powersemiconductor device module of claim 1, wherein the branched networkextends through a first plurality of spring loaded contact pins, andwherein the main emitter terminal is coupled through no spring loadedcontact pin to the emitter pad of any of the semiconductor device dice.3. The power semiconductor device module of claim 2, further comprising:a gate terminal, wherein the gate terminal is coupled through a secondplurality of spring loaded contact pins to the gate pads of thesemiconductor device dice.
 4. The power semiconductor device module ofclaim 1, wherein the main emitter terminal is a surface of the bottomplate member.
 5. The power semiconductor device module of claim 1,wherein the main emitter terminal is a surface of a circular bottom lid,and wherein the bottom lid is attached to the bottom plate member. 6.The power semiconductor device module of claim 1, wherein thesemiconductor device dice are Insulated Gate Bipolar Transistor (IBGT)device dice.
 7. The power semiconductor device module of claim 1,wherein the power semiconductor device module comprises a PrintedCircuit Board (PCB), and wherein the branched network extends throughthe PCB.
 8. The power semiconductor device module of claim 1, furthercomprising: a plurality of insulative four-sided frames, wherein each ofthe four-sided frames is disposed around a corresponding one of theplurality of pedestals.
 9. The power semiconductor device module ofclaim 1, wherein the power semiconductor device module is a disc-shapedpress pack module.
 10. A power semiconductor device module comprising: atop plate member; a plurality of pedestals; a plurality of semiconductordevice dice, wherein each of the semiconductor device dice is positionedabove a corresponding one of the plurality of pedestals between thepedestal and the top plate member, and wherein each of the semiconductordevice dice has an emitter pad and a gate pad; a main emitter moduleterminal; an auxiliary emitter module terminal; first means for couplingthe main emitter module terminal through the pedestals to the emitterpads of the semiconductor device dice; and second means for coupling theauxiliary emitter module terminal through a conductive path to theemitter pads of the semiconductor device dice, wherein the conductivepath includes a plurality of contact pins, and wherein the conductivepath does not extend through any of the pedestals.
 11. The powersemiconductor device module of claim 10, wherein the first meanscomprises no contact pins, and wherein the second means comprises aprinted circuit board, a plurality of resistors, and a plurality ofshims in addition to the plurality of contact pins.
 12. The powersemiconductor device module of claim 10, wherein the semiconductordevice dice are Insulated Gate Bipolar Transistor (IBGT) device dice.13. The power semiconductor device module of claim 10, furthercomprising: a plurality of insulative four-sided frames, wherein each ofthe four-sided frames is disposed around a corresponding one of theplurality of pedestals.
 14. The power semiconductor device module ofclaim 10, wherein the power semiconductor device module is a disc-shapedpress pack module.
 15. A power semiconductor device module comprising: atop plate member; a bottom plate member having a plurality of pedestals;a plurality of semiconductor device dice, wherein each of thesemiconductor device dice is positioned above a corresponding one of theplurality of pedestals between the bottom plate member and the top platemember, and wherein each of the semiconductor device dice has an emitterpad and a gate pad; an auxiliary emitter terminal, wherein the auxiliaryemitter terminal is coupled via a branched network to the emitter padsof the semiconductor device dice; a main emitter terminal, wherein themain emitter terminal is coupled through the pedestals to the emitterpads of the semiconductor device dice, and wherein the branched networkdoes not extend through any of the pedestals; and a plurality ofinsulative four-sided frames, wherein each of the four-sided frames isdisposed around a corresponding one of the plurality of pedestals,wherein each of the four-sided frames has a first channel and a secondchannel, wherein a first spring-loaded contact pin is disposed in thefirst channel, and wherein a second spring-loaded contact pin isdisposed in the second channel.
 16. The power semiconductor devicemodule of claim 15, wherein the first spring-loaded contact pin contactsthe gate pad of one of the semiconductor device dice disposed in theframe, wherein the second spring-loaded contact pin contacts a shimdisposed in the frame, and wherein the shim is in physical contact withthe emitter pad of the one of the semiconductor device dice.
 17. A powersemiconductor device module comprising: a top plate member; a bottomplate member having a plurality of pedestals; a plurality ofsemiconductor device dice, wherein each of the semiconductor device diceis positioned above a corresponding one of the plurality of pedestalsbetween the bottom plate member and the top plate member, and whereineach of the semiconductor device dice has an emitter pad and a gate pad;an auxiliary emitter terminal, wherein the auxiliary emitter terminal iscoupled via a branched network to the emitter pads of the semiconductordevice dice; and a main emitter terminal, wherein the main emitterterminal is coupled through the pedestals to the emitter pads of thesemiconductor device dice, wherein the branched network does not extendthrough any of the pedestals, wherein a first conductive path extendsfrom a gate terminal of the power semiconductor device module, andthrough a first spring-loaded contact pin, and to the gate pad of one ofthe semiconductor device dice, and wherein a second conductive pathextends from the auxiliary emitter terminal, and through a secondspring-loaded contact pin, through a shim, and to the emitter pad of oneof the semiconductor device dice.